Q - What is an impeller key?
A - An impeller key is a square or rectangular piece of metal located in the
keyway underneath the impeller hub and the shaft. The function of a key is
two-fold: (1) to provide a positive lock to prevent any angular displacement
between the impeller and the shaft, and (2) to transmit input power from the shaft
into the impeller.
A key is subjected to both shearing and bearing (or compressive) stresses.
Common key materials are bronze, carbon steel, and stainless steel.
Q - Why is a key with rounded ends preferable over a key with square ends
that is cheaper and easier to machine?
A - This has something to do with machining the keyway on the shaft. Because
of the tooling used, the keyway will always have rounded ends, except when the
keyway is located at the very end of the shaft (as is typical in an overhang pump) in
which case only one end is rounded. Therefore, a square key will not fill up all the
spaces in the keyway - the rounded ends will remain void. This void on one side of
the shaft will create some unbalance on the rotor that may result in excessive
vibration specially in high horsepower, or high speed, pumps